1. Field of the Invention
The invention relates to an apparatus and process for using corona discharge to deposit colloidally suspended molecules onto substrate surfaces. The method is applicable to deposition of organic and inorganic compounds, particularly to proteins and related biological compounds of interest onto selected substrates with little or no loss of native structure or activity.
2. Description of Background Art
There is increasing interest in the immobilization of biologically active substances onto various substrates without significant alteration of function or desired activity. Surfaces coated with antibiotics, for example, are typically prepared by dipping or paint processes, which often result in poor adhesion, incomplete surface wetting or poor adhesion.
Ionic plasma deposition (IPD) methods have been extensively developed and used in coating processes, predominately with the objective of producing highly adhesive coatings and customized surface characteristics. Attention has recently focused on preparing coated surfaces that are biocompatible, such as those suitable for medical implants where the coatings enhance cell adhesion or where antimicrobial coatings are important in avoiding potential sepsis after surgery.
Corona discharge is a well-known phenomenon which has long been observed in nature and traditionally used in a number of commercial and industrial processes. It is currently used in ozone production, control of surface generated electrical charges and in photocopying. Electric corona discharge has also been used to modify surfaces, particularly for plastic articles to improve surface characteristics, as described in U.S. Pat. No. 3,274,089. An electrostatic coating process involving a corona discharge of a liquid or powdered material is described as a coating method, U.S. Pat. No. 4,520,754.
A corona is generated when the potential gradient is large enough at a point in the fluid to cause ionization of the fluid so that it becomes conductive. If a charged object has a sharp point, the air around that point will be at a much higher gradient than elsewhere. Air near an electrode can become ionized (partially conductive), while regions more distant do not. When the air near the point becomes conductive, it has the effect of increasing the apparent size of the conductor. Since the new conductive region is less sharp, the ionization may not extend past the local region. Outside the region of ionization and conductivity, the charged particles slowly find their way to an oppositely charged object and are neutralized.
Corona discharge usually involves two asymmetric electrodes; one highly curved, e.g., a needle tip or a small diameter wire, and one of low curvature, e.g., a plate, or a ground. The high curvature ensures a high potential around the electrode, providing for the generation of a plasma. If the geometry and gradient are such that the ionized region continues to grow instead of stopping at a certain radius, a completely conductive path may be formed, resulting in a momentary spark or a continuous arc.
Coronas may be positive or negative. This is determined by the polarity of the voltage on the highly-curved electrode. If the curved electrode is positive with respect to the flat electrode a positive corona exists; otherwise the corona is negative. The physics of positive and negative coronas are strikingly different. This asymmetry is a result of the great difference in mass between electrons and positively charged ions, with only the electron having the ability to undergo a significant degree of ionizing inelastic collision at ordinary temperatures and pressures.
Corona discharge systems have been used to activate chemical compounds, generally to deposit polymers and polymerizable monomers formed within a corona discharge onto surfaces as protective coatings; as described in U.S. Pat. No. 3,415,683. A corona discharge reactor for chemically activating constituents of a gas stream; e.g, sulfur and nitrogen oxides and mercury vapor, is described in U.S. Pat. No. 5,733,360. The reactor is designed to pulse generate a corona by applying high voltages pulses for up to 100 nanoseconds to a plurality of corona discharge electrodes.
WO 2006/046003 describes several methods for coating substrates involving use of a plasma, including use of low pressure pulsed plasma to introduce monomers or monomers in combination with free radical initiators to initiate polymerization on a suitable substrate. An atmospheric pressure diffuse dielectric barrier discharge assembly is used into which an atomized liquid containing the monomers is introduced so that a coating material is formed from atomized drops of from 10 to 100 μm. An atmospheric pressure glow discharge plasma generating apparatus using radiofrequency energized electrodes is described in WO 03/084682.
A plasma coating apparatus and method are described in WO 02/28548. Liquid or solid atomized coating forming materials are introduced into a plasma discharge at atmospheric pressure and are useful for organic coatings such as polyacrylic acid or perfluoro compounds in addition to silicon-containing monomers.
Corona effects are not always considered beneficial and may in fact cause arcing, or the breakdown of the corona. In addition to this breakdown, the corona effect may be too strong to successfully only singly charge a complex molecule. When molecules ionize at a higher level, they may break apart and lose structural and functional properties.
One disadvantage of depositing materials generated in plasmas from liquid solutions is that any solvent present is typically deposited along with the intended material, creating unintended structures. For most processes where corona discharge can occur during plasma generation, efforts are usually taken to reduce the corona effect rather then using this effect as a deposition technique.
Deficiencies in the Art
The loss of functional and/or physical properties of plasma surface deposited organic molecules points to the need to develop methods of maintaining desirable biological activities of immobilized materials. Attempts to engineer biological coatings on a range of substrates, such as plastics, metals, polymers, and ceramics have met with limited success and generally have failed to deposit biologically active agents on surfaces without compromising desired activity.